As is well known, the coefficient of static friction is a proportionality constant, f, relating the maximum amount of force required to translate one body relative to another body to cause incipient relative movement between the bodies (the maximum force of static friction, (F), to overcome a force (N) normal to the direction of movement and is mathematically given by f = F/N.
Prior art methods and apparatus for determining the static coefficient of friction between a plurality of bodies having contacting surfaces have involved connecting a relatively inflexible filament to one of the bodies and applying a constant force to the filament, whereby a constant force is applied to the one body. The bodies are loaded with a normal force of predetermined magnitude, usually by mounting the bodies horizontally and placing a weight on the upper body. The force on the filament at the moment of incipient movement between the contacting surfaces is determined to enable the static coefficient of friction to be calculated. After the incipient movement, movement of one of the bodies relative to the other body at constant speed is continued by pulling the inflexible cable at constant speed. The force on the inflexible cable is determined to enable the kinetic coefficient of friction to be determined. Hence, this standard procedure, which is reported in the 1970 Annual Book of ASTM Standards, pages 544-548, is not suitable for quickly and easily making a series of tests to determine the static coefficient of friction. If it is desired to conduct a series of tests to determine static friction so that increased reliability of the static friction determination can be obtained, it is necessary with the prior art technique and apparatus to set up the apparatus multiple times. Of course, it is undesirable to set up an apparatus a number of times in order to take multiple tests because of the time and expense involved, as well as because of the possibility of unreliable measurements between different setup procedures.
It is, accordingly, an object of the present invention to provide a new and improved method of and apparatus for determining the static coefficient of friction between contacting surfaces of a plurality of bodies.
A further object of the invention is to provide a new and improved method of and apparatus for enabling the coefficient of static friction between contacting surfaces of a plurality of bodies to be determined from a series of measurements which require only a single setup of the testing equipment.
Prior art devices and methods for testing the static coefficient of friction have generally been designed for determining coefficients of friction under light compressive loads. However, there are certain situations wherein the coefficient of the friction must be determined for extremely heavy loads. We have found that the normal load provided by a weight, as indicated by the prior art test apparatus and methods, does not provide the required stress levels.
It is, accordingly, a further object of the invention to provide a new and improved apparatus for and method of testing for the coefficient of static friction in situations wherein the coefficient is considerably in excess of one.
Another object of the invention is to provide a new and improved method of and apparatus for enabling static coefficient of friction tests to be determined at stress levels on the order of 600 p.s.i.